Investigation of Process Parameters of Phosphogypsum for Preparing Calcium Sulfoaluminate Cement

Abstract

Preparing calcium sulfoaluminate cement (CAS) from solid waste phosphogypsum (PG) instead of natural gypsum is an effective way to utilize solid waste. In this paper, CAS clinker was successfully prepared from PG and the mineral content of calcium sulfoaluminate (${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$) in the sample was above 65%. The effects of raw material ratio, calcination temperature, and time on clinker composition were investigated. The mechanical properties of different samples were tested. The optimum ratio for preparing CAS using PG was 42.23% limestone, 17.43% PG, and 40.34% bauxite. The optimal calcination conditions are a high temperature of 1250 °C for 45 min. The 3-day compressive strengths of the laboratory-prepared CAS were all above 50 MPa. It was found that as the calcination temperature increased, the amount of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ produced gradually increased. Temperatures above 1300 °C resulted in the decomposition of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$. The calcination time did not significantly affect the mineral composition of the clinker or the strength of the cement. ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ was observed to be rounded and hexagonal platelets with crystal sizes of 1 to 2 μm, a relatively small size that is favorable to the hydration of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$, as observed by SEM images. In addition, the high calcination temperature affected the particle morphology of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$, changing it from a well-defined polygonal structure to a molten state. The test results provide helpful information for improving PG utilization and applying PG in CAS production.

1. Introduction

Phosphogypsum (PG), consisting of calcium sulfate dihydrate and some impurities (sulfuric acid, phosphoric acid, phosphide, fluoride, and organic matter), is a solid waste from the wet phosphoric acid process [1,2]. Due to the development of the phosphate industry, PG emissions are increasing. Annual PG emissions worldwide are 280 million tonnes, of which China emits about 80 million tonnes annually [3]. No practical method has been found to treat PG, and less than 10% of it is used to prepare ammonium sulfate, soil modifiers, gypsum gelling materials, or silicate cement retarders [4,5,6]. Untreated PG exists in large piles on vacant land, which not only occupies land, but pollutes the ecological environment, poses a significant safety hazard, and restricts the development of the phosphate industry [7,8]. Therefore, the question to be addressed is how to deal with PG.

Calcium sulfoaluminate cement (CAS) is a hydraulic gel material with excellent properties such as fast hardening, early strength, low alkalinity, frost resistance, seepage resistance, and corrosion resistance. CAS is made by mixing a certain proportion of gypsum, bauxite, and limestone as raw materials, calcining at a high temperature to form a clinker with calcium sulfoaluminate (${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$) and dicalcium silicate (C₂S) as the primary minerals, and then adding an appropriate proportion of gypsum [9,10,11]. CAS has a low lime saturation factor and requires less limestone to prepare the raw material for cement, so it emits relatively less carbon dioxide [12,13,14]. In addition, the calcination temperature of sulfate aluminate cement clinker is about 150 °C lower than that of ordinary silicate cement clinker [15,16,17], and the fuel consumed during calcination is lower, making calcium sulfoaluminate cement a low-consumption and low-carbon-emission cementitious material that is of increasing interest to companies and researchers [18,19].

Currently, China has many PG emissions and large stockpiles, and there is a huge potential problem of environmental pollution [20]. Moreover, with the rapid development of urban and rural construction, the demand for special cement is also increasing [21,22]. The content of gypsum dihydrate in PG is above 85%, similar to the main component of natural gypsum [23]. The use of PG to replace natural gypsum in the preparation of CAS has become a popular topic for research [24], and it is essential to improve the utilization rate of PG and reduce the cost of CAS cement. However, few comparative investigations exist on the preparation of sulfate aluminate cement from PG.

Shen et al. successfully produced belite sulfoaluminate-ternesite cement in the laboratory using PG by secondary heat treatment with low early hydration strength [25]. Huang et al. investigated the effects of impurities of phosphoric acid, dicalcium phosphate, and calcium phosphate on CAS. The results showed that phosphoric acid and phosphate adversely affected CAS’s hydration and mechanical properties, with phosphoric acid having the most significant effect [26]. Zhao et al. used density functional theory (DFT) simulations to demonstrate that P atoms were more likely to replace S atoms to form phosphorus-doped ye’elimite when PG was used to produce CAS clinker [27]. Previous studies have focused on the hydration of PG-prepared CAS clinker’s hydration characteristics, properties, and doping mechanism [28,29,30]. However, no systematic research has been conducted on the preparation process of CAS produced by PG, which is of great importance for industrial production.

This study prepared CAS clinker from PG, bauxite, and limestone by controlling the process parameters. The mineral composition and compressive hydration strength of CAS clinker were studied under different conditions to obtain the optimal preparation process. Various raw materials ratios were calculated based on their composition and calcined to obtain CAS clinker with different mineral compositions. The optimum ratio of raw materials was determined by analyzing the clinker mineral composition and cement compressive strength. The clinker calcination process was optimized by controlling the calcination temperature and time. The optimum calcination conditions were determined according to the composition of the clinker minerals and the tested mechanical properties. A dosage scheme and process parameter model for CAS cement clinker with PG was established as the primary raw material.

2. Experimental Protocol

2.1. Raw Materials

This paper’s bauxite, PG, and limestone were all sourced from Guizhou Urnford Group. All raw materials were ground by a planetary ball mill and passed through an 80 μm square hole sieve with less than a 5% residue rate. The chemical composition of the raw materials is listed in

Table 1. Chemical compositions of the materials (wt.%).

Materials	SO3	CaO	SiO2	Al2O3	Fe2O3	TiO2	MgO	P2O5	F	LOI
PG	50.33	39.78	3.61	0.28	0.37	0.08	-	0.83	0.67	4.05
Limestone	0.16	88.68	3.55	0.94	0.71	-	4.39	-	-	1.57
Bauxite	5.80	0.45	9.60	69.56	4.74	5.47	0.18	-	-	4.2

, and the X-ray diffractometer (XRD) analysis of PG, bauxite, and limestone is shown in Figure 1. The XRD pattern shows that the main mineral phases of PG are gypsum dihydrate and quartz. The main mineral phases of limestone are calcite and dolomite, and the primary mineral phases of bauxite are alumina, quartz, and hematite.

Figure 2 shows the particle size distribution profile of the three raw materials after grinding. The finer the raw material, the larger the specific surface area, and the faster the raw material reacts to produce clinker minerals during calcination. Therefore, the particle size of the three raw materials was measured using a laser diffraction particle size analyzer (NKT6200, Jinan, China). As shown in Figure 2, the particle size distribution of PG is more concentrated, mainly around 70 μm. In contrast, the particle size distribution of bauxite and limestone is broader, but the fineness of all three raw materials is controlled to pass through an 80 μm square hole sieve with a sieve margin of less than 5%. The target phase combinations for CAS clinker are shown in

Table 2. Design mineral composition of cement clinker (wt.%).

Sample	${\mathbf{C}}_4{\mathbf{A}}_3\overline{S}$	C4AF	C2S	$\mathbf{C}\overline{S}$
S1	74	8	18	0
S2	69	8	18	5
S3	64	8	18	10
S4	59	8	18	15

. Based on the target phase combinations and the chemical composition in Table 1, the ratios for CSA cement raw materials were determined, as shown in

Table 3. Proportioning of the cement raw materials (wt.%).

Sample	Limestone	Phosphogypsum	Bauxite
S1	45.28	12.15	42.57
S2	42.23	17.43	40.34
S3	39.44	22.67	37.89
S4	37.23	27.32	35.45

. The ground raw materials were accurately weighed and homogenized according to the ratios in Table 3. The raw material mixture was mixed with 10 wt.% water and pressure molded. After drying in a 40 °C oven, it was placed in a high-temperature furnace for firing. The firing process is shown in Figure 3. After calcination, the clinker was removed for rapid cooling. The samples were cooled and placed in a planetary ball mill for 5 min, and then ground finely in an agate mortar to pass through an 80 µm sieve. Analytical tests were carried out to determine the optimum raw material ratio, different calcination processes were used to prepare the cement clinker, and the effect of calcination temperature and time on the minerals and compressive strength of the cement clinker was investigated. The calcination conditions for different samples are shown in

Table 4. Calcination conditions of samples.

Sample	Calcination Temperature	Calcination Time
T1	1150 °C	45 min
T2	1200 °C	45 min
T3	1250 °C	45 min
T4	1300 °C	45 min
D1	1250 °C	30 min
D2	1250 °C	45 min
D3	1250 °C	60 min
D4	1250 °C	75 min

.

2.2. Test Methods

The thermal gravimetric (TG) tests were carried out using an integrated heat analyzer (Mettler-Toledo TGA/DSC1, Switzerland) in a dry nitrogen environment at 50 mL/min. The dried raw materials were ground into powders with a diameter of less than 80 μm, mixed homogeneously according to the ratios shown in Table 3 and placed in an alumina crucible, and ramped up from 50 to 1300 °C at a rate of 10 °C/min. The DTG curve was obtained by solving the TG curve in the first order. The DTG curve reflects the decomposition peaks corresponding to the decomposition of the different phases.

The calcined cement clinker was ground and passed through an 80 μm sieve for XRD (Bruker D8 Advance, German) analysis. The instrument emission light source was a copper target (λ = 0.154 nm) with an operating voltage of 40 kV and an operating current of 40 mA. The test range was 5–80° (2θ) with a single step scan time of 0.5 s at a step size of 0.02°. The XRD profiles were first qualitatively analyzed using EVA software to determine the type of physical phase of the samples to be tested. This was followed by a quantitative analysis of the clinker mineral composition of the different samples by the Rietveld method (error 2%) using TOPAS 4.2 software [31]. The parameters fitted during the quantitative analysis were the background coefficient, zero displacement error, phase scale, cell parameters, phase shape parameters, and the merit orientation factor [32]. Due to a large amount of amorphous material in the cement samples, the tests were performed using the standard internal method, where 15 wt.% ZnO was added to the samples as an internal standard to determine the amount of ACn (containing amorphous phases, which are poorly crystallized and undetected by XRD) [33,34]. The Rietveld refinement process uses the Rwp value as an evaluation indicator for the fit results’ reliability; usually, when Rwp < 15%, the results are considered reliable [35,36,37].

CAS clinker and gypsum dihydrate were mixed and ground at a molar ratio of 1:1.5 to obtain CAS cement, which was poured into a mixer at a water–cement ratio of 0.5 and mixed for two minutes at a controlled speed of 110–120 r/min to obtain a net cement paste. Cubic specimens were prepared using a 20 mm × 20 mm × 20 mm mold, initially placed at a temperature of (20 ± 1) °C and (95 ± 1)% humidity in a standard conditioning room. After demolding, the cubic specimens were cured in a standard curing chamber. The test cubes were tested for compressive strength using a microcomputer-controlled electronic universal testing machine (WDW-50, China). All test results were averaged from three sets of tests, and provide the characteristic values of compressive strength with standard deviation.

The microstructure of the cement clinker samples was observed by scanning electron microscopy (FEI NOVA Nano-SEM 450, USA). The accelerating voltage was 3 or 6 kV, and the electron beam’s spot size was 3.0. The samples were gold coated for 30 s and then placed in the instrument for testing.

3. Results and Discussion

3.1. Thermal Analysis

Figure 4 shows the thermal analysis curves of cement raw materials at different ratios. As shown in Figure 4a, there are several weight loss peaks in the thermogravimetric curve. The first weight loss peak occurs at 150–170 °C; mainly, the raw material undergoes a dehydration reaction, including physical and crystalline water. The second corresponding weight loss peak occurs at 490–510 °C, mainly when the bauxite undergoes a decomposition reaction, forming Al₂O₃ and SiO₂. The third occurs at 700–800 °C, where the decomposition reaction of limestone occurs at this stage, producing CaO and CO₂, reacting as in Equation (1) [29]. The last occurs at 900 to 1030 °C and is due to the decomposition of PG, as shown in Equation (2) [38,39].

$${\mathrm{CaCO}}_3\to \mathrm{CaO}+{\mathrm{CO}}_2\uparrow$$

(1)

$$2{\mathrm{CaSO}}_4\to 2\mathrm{CaO}+2{\mathrm{SO}}_2\uparrow +{\mathrm{O}}_2\uparrow$$

(2)

$$2\mathrm{CaO}+{\mathrm{Al}}_2{\mathrm{O}}_3+{\mathrm{SiO}}_2\to 2\mathrm{CaO}\cdot {\mathrm{Al}}_2{\mathrm{O}}_3\cdot {\mathrm{SiO}}_2$$

(3)

$$3\mathrm{CaO}+3{\mathrm{Al}}_2{\mathrm{O}}_3+{\mathrm{CaSO}}_4\to 3\mathrm{CaO}\cdot 3{\mathrm{Al}}_2{\mathrm{O}}_3\cdot {\mathrm{CaSO}}_4$$

(4)

$$3\mathrm{CaO}+3\left(2\mathrm{CaO}\cdot {\mathrm{Al}}_2{\mathrm{O}}_3\cdot {\mathrm{SiO}}_2\right)+{\mathrm{CaSO}}_4\to 3\mathrm{CaO}\cdot 3{\mathrm{Al}}_2{\mathrm{O}}_3\cdot {\mathrm{CaSO}}_4+3\left(2\mathrm{CaO}\cdot {\mathrm{SiO}}_2\right)$$

(5)

$$2\mathrm{CaO}+{\mathrm{SiO}}_2\to 2\mathrm{CaO}\cdot {\mathrm{SiO}}_2$$

(6)

$$2\left(2\mathrm{CaO}\cdot {\mathrm{SiO}}_2\right)+{\mathrm{CaSO}}_4\to 4\mathrm{CaO}\cdot 2{\mathrm{SiO}}_2\cdot {\mathrm{CaSO}}_4$$

(7)

$$4\mathrm{CaO}\cdot 2{\mathrm{SiO}}_2\cdot {\mathrm{CaSO}}_4\to 2\left(2\mathrm{CaO}\cdot {\mathrm{SiO}}_2\right)+{\mathrm{CaSO}}_4$$

(8)

As the temperature continues to rise, 2CaO-Al₂O₃-SiO₂ begins to form, and when it increases to 1000 °C, ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ and C₂S gradually develop, and the main chemical reactions that occur during the formation process are shown in Equations (3)–(8) [18,40]. Therefore, the comprehensive reaction process above is divided into three main stages. First is the stage of dehydration and decomposition of raw materials. The second is the generation stage of the intermediate mineral phase, mainly including intermediate products such as C₂AS and CA. The final stage is the generation stage of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$, which is primarily the reaction between the intermediate mineral phases. The thermal analysis of the cement raw meal indicated that the cement clinker was produced at a temperature of 1100 to 1300 °C. Based on this analysis, the next section of the raw meal was prepared by continuous calcination at 1250 °C for 60 min, which also provided a reference basis for further determination of the optimum calcination process.

3.2. Determination of CAS Raw Material Ratios

3.2.1. Clinker Mineral Composition Analysis

Figure 5 shows the XRD patterns of the cement clinker with different ratios. XRD curves have distinctive peaks characteristic of the homogenous minerals, mainly ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$, C₂S, C₂AS, and C$\overline{\mathrm{S}}$. From S1 to S4, the diffraction peaks of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ in the prepared cement clinker gradually decrease as the ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ content in the design solution decreases. Quantitative analysis of the cement clinker minerals was carried out to determine the content of each phase in the clinker. The results of the Rietveld quantitative analysis are shown in

Table 5. Contents of cement clinker minerals (wt.%).

Sample	Phase Content (wt.%)					Rwp (%)
	${\mathbf{C}}_4{\mathbf{A}}_3\overline{S}$	C2S	C4AF	$\mathbf{C}\overline{S}$	C2AS
S1	76.68	10.30	3.24	-	9.78	12.17
S2	76.00	12.55	3.95	2.29	5.21	11.40
S3	73.00	13.54	5.42	5.26	2.78	12.76
S4	66.67	15.81	3.98	10.80	2.74	12.33

. The ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ content was high in all four groups of samples and exceeded 65%, with the highest content reaching 76.68%. The crystalline structure of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ is a porous skeleton consisting of connected aluminum-oxygen tetrahedra, which are highly reactive due to their porous structure and can be rapidly hydrated in the presence of gypsum generating ettringite (AFt), thus improving its early compressive strength [41,42,43]. It is therefore hypothesized that adding a certain amount of gypsum dihydrate to the four groups of cement clinker will result in a high content of calcium sulfoaluminate, leading to faster development of cement strength and higher early strength in all cases. C₂S and C$\overline{\mathrm{S}}$ in the clinker are lower than the design value. Due to the decomposition of PG during calcination, the actual residual amount of calcium sulfate is less than the design value. Analyzing the results of the S1 group of tests, there is no residual C$\overline{\mathrm{S}}$ in the design scenario, which exactly meets the demand for ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ mineral production. However, it can be seen from the quantitative results that when the PG decomposed, the remaining C$\overline{\mathrm{S}}$ could not meet the need for the conversion of 2CaO-Al₂O₃-SiO₂ (C₂AS) to ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ and C₂S, resulting in the clinker minerals containing the intermediate mineral C₂AS. Thus, the C₂S and C$\overline{\mathrm{S}}$ content in each group of clinker minerals was less than the design amount, and the extra PG dispensed into the raw meal was converted to high-temperature hard gypsum. It can also be seen that the C₂S content in the S1–S4 samples gradually increases. Under the presence of gypsum, the rate of hydration of C₂S develops slowly and gradually increases in the later stages. Therefore, it can be assumed from this that the compressive strength of S4 develops relatively well in the later stages compared to S1.

The microscopic morphology of Group S2 cement clinker was analyzed by SEM photographs, as shown in Figure 6. It can be seen from the photographs that the clinker is mostly calcium sulfate aluminate minerals with crystal sizes of about 1 to 2 μm, in the form of hexagonal plates or fine rounded crystals with smooth surfaces, and closely arranged with uniform sizes between the particles. A large amount of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ can react quickly to produce ettringite (AFt) during hydration, which causes the cement test block to obtain high early compressive strength.

3.2.2. Compressive Strength

Figure 7 shows the compressive strength of the cement test blocks for four different ratios. It can be seen from the figure that the early compressive strength follows the same trend as the content of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ minerals in the samples and decreases with the decrease in ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ minerals. The early strength of the four groups of samples mainly originates from the hydration of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ and the generation of AFt. At 1 day of age, S1 had the highest compressive strength of 44.8 MPa, and S4 had the lowest compressive strength of 40 MPa. After 3 days of hydration, S2 had the highest compressive strength of 55.5 MPa, followed by S1 at 54.2 MPa. The late compressive strength development of the samples was relatively slow, probably because of the low C₂S mineral content. In summary, the S2 specimens showed the best strength development and did not show strength inversion at the later stage.

Combined with the mineral composition, the ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ content is high, and thus the hydration activity is good. Through the rapid hydration of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ with hard gypsum in the early stages of hydration, it has good early compressive strength, reaching 44.8 MPa at 1 d. In this test, S2 can be chosen as the best mix ratio design, and the effect of different calcination processes on the clinker was determined later. In addition to this, when looking at the compressive strength of the test blocks in group S4, although the compressive strength was lower than that of the other three groups, the compressive strength grew fastest in the later stages, which verifies that group S4 had the highest C₂S content in the cement clinker and the fastest growth in compressive strength in the later stages when the cement clinker was quantified.

3.3. Calcination Process Optimisation

3.3.1. Calcination Temperature

Figure 8 shows the XRD patterns of the S2 samples at different calcination temperatures. The diffraction peaks of C$\overline{\mathrm{S}}$ and C₂AS are obvious at 1150 °C, and the diffraction peak of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ is relatively weak, so it can be seen that the firing process has not been entirely completed. There are still transitional phases of C₂AS and C$\overline{\mathrm{S}}$ in the clinker, which has not been completely transformed into ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ and C₂S. Accordingly, the transitional phases in the cement clinker can be used to determine the calcination temperature of the clinker and to judge whether the clinker reaction process is completed. When the calcination temperature rises from 1150 to 1250 °C, the diffraction peak of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ gradually increases, and when the calcination temperature continues to grow to 1300 °C, the diffraction peak of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ decreases. According to this, it can be seen that when the temperature rises to 1300 °C, ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ begins to decompose, and the reaction equation is shown in Equations (9) and (10) [44]. An observation of the clinker will show that the clinker center produces molten lumps with a yellow-brown color, indicating that the calcination temperature is high. Figure 9 shows the pictures of experimental samples before and after high-temperature calcination.

$$2\left(4\mathrm{CaO}\cdot 3{\mathrm{Al}}_2{\mathrm{O}}_3\cdot {\mathrm{SO}}_3\right)\to 6\left(\mathrm{CaO}\cdot {\mathrm{Al}}_2{\mathrm{O}}_3\right)+2\mathrm{CaO}+2{\mathrm{SO}}_2\uparrow +{\mathrm{O}}_2\uparrow$$

(9)

$$10\left(4\mathrm{CaO}\cdot 3{\mathrm{Al}}_2{\mathrm{O}}_3\cdot {\mathrm{SO}}_3\right)\to 16\left(\mathrm{CaO}\cdot {\mathrm{Al}}_2{\mathrm{O}}_3\right)+2\left(12\mathrm{CaO}\cdot 7{\mathrm{Al}}_2{\mathrm{O}}_3\right)+10{\mathrm{SO}}_2\uparrow +5{\mathrm{O}}_2\uparrow$$

(10)

The mineral content of the S2 samples at different calcination temperatures shown in

Table 6. Contents of S2 samples fired at different temperatures (wt.%).

Sample	Phase Content (wt.%)							Rwp (%)
	${\mathbf{C}}_4{\mathbf{A}}_3\overline{\mathbf{S}}$	C2S	C4AF	$\mathbf{C}\overline{\mathbf{S}}$	C2AS	C12A7	CA
T1	58.67	13.30	5.24	8.34	14.45	-	-	9.15
T2	67.34	14.53	4.97	5.78	7.38	-	-	11.78
T3	72.89	15.48	5.62	3.72	2.29	-	-	10.40
T4	68.56	14.81	5.15	2.45	-	3.25	5.78	12.11

was quantified by the Rietveld method. At a calcination temperature of 1150 °C, the clinker contains less ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ at 58.67%, while it contains 14.45% C₂AS and 8.34% C$\overline{\mathrm{S}}$, indicating that the calcination temperature is insufficient and the reaction is not complete. As the temperature increased, the ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ content gradually increased, and the intermediate mineral C₂AS disappeared. When the temperature rose to 1300 °C, the ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ content dropped to 68.56%, while a small amount of C₁₂A₇ and CA appeared in the clinker when the temperature was too high, and ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ decomposed. Therefore, the calcination temperature of CAS clinker was determined to be 1250 °C to ensure the adequate formation of clinker minerals and prevent over-burning.

The compressive strength of CAS at different calcination temperatures is shown in Figure 10. When the calcination temperature is increased from 1150 to 1250 °C, the compressive strength of the cement shows an increasing trend with increasing calcination temperature. In contrast, when the calcination temperature rises to 1300 °C, the cement’s compressive strength decreases slightly, which is related to the clinker’s mineral composition. The cement compressive strength is in good agreement with the XRD analysis, and the calcination temperature of 1250 °C was chosen for the firing of the clinker.

Figure 11 shows the microscopic morphology of S2 at different calcination temperatures. Comparing Figure 11a–c, it is found that the morphological characteristics of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ are the same when the calcination temperature is 1150 to 1250 °C. The size of the fired ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ particles are about 1 to 2 μm, with a polygonal structure and precise outline, and the inter-particle boundary is clear. When the temperature was increased to 1300 ℃, the morphological characteristics of the particles in Figure 11d show that most of the ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ particles had no apparent outline shape, and the particles were fused without obvious boundaries. Therefore, when the calcination temperature is too high, it will affect the particle morphology of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$, and changing it from a well-defined polygonal structure to a molten state will affect the cement’s hydration reaction.

3.3.2. Calcination Time

Figure 12 shows the XRD patterns of the S2 sample calcined at 1250 °C at different times. It can be seen from the graph that the intensity of the diffraction peaks of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ increased when the calcination time was increased from 30 to 45 min, indicating that the increase in time favored the formation of clinker minerals. The increase in time from 45 to 75 min did not result in any significant change in the diffraction peaks of the cement clinker minerals, indicating that calcination time within this range did not significantly affect mineral formation. It can be assumed that the clinker minerals were already formed after 45 min of calcination at a high temperature, and there was no need to extend the calcination time further. Therefore, in terms of the mineral composition of the cement clinker, 45 min can be chosen as the optimum time for the calcination of the CAS clinker.

Table 7. Contents of S2 samples fired at different times (wt.%).

Sample	Phase Content (wt.%)						Rwp (%)
	${\mathbf{C}}_4{\mathbf{A}}_3\overline{\mathbf{S}}$	C2S	C4AF	$\mathbf{C}\overline{\mathbf{S}}$	C2AS	CA
D1	69.01	13.30	5.08	6.50	6.11	-	12.65
D2	72.78	14.52	5.68	3.77	3.25	-	11.17
D3	73.14	15.24	5.77	3.79	-	2.06	13.34
D4	72.24	15.71	5.49	2.68	-	3.88	10.38

shows the content of each mineral in the S2 samples for different calcination times. It can be seen that the D1 group contains less ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$, and the other three groups have similar ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ content in the clinker with no significant changes. At 75 min, there is a tendency for the ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ to decrease slightly. At the same time, a CA of 3.88% occurs, indicating that the calcination time of the clinker should not be too short, but when the calcination time is too long, a small amount of decomposition of the ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ in the clinker will also occur.

The effect of different calcination times on the compressive strength of the cement is shown in Figure 13. The lowest cement compressive strength is evident at a calcination time of 30 min; samples calcined for 45, 60, and 75 min show essentially similar cement compressive strengths with no significant difference, thus further validating the results of the XRD pattern and quantitative analysis, indicating that the clinker minerals formed mainly within 45 min of calcination at high temperatures, with no significant change in strength when extended to 75 min. Therefore, the clinker mineral formation and cement compressive strength results indicate that the optimum holding time for cement clinker calcination is 45 min.

4. Conclusions

This study focused on the preparation process and properties of CAS prepared from PG; investigated the control of firing process parameters of CAS clinker; analyzed the influence of raw material ratio, calcination temperature, and time on the mineral type and content of clinker; and established the dosage scheme and firing process model of CAS clinker with PG as the main raw material. The main conclusions are shown below.

(1) The optimum ratio for preparing CAS clinker in this experiment was 42.23% limestone, 17.43% PG, and 40.34% bauxite. The compressive strength of CAS specimens at 1, 3, and 28 days could reach 42.8, 55.5, and 60.9 MPa, respectively.

(2) The optimum process parameter for preparing CAS using PG is calcination at 1250 °C for 45 min. Below 1200 °C, ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ is not yet fully formed, and many transitional phases of C₂AS and $\mathrm{C}\overline{\mathbf{S}}$ are still present in the clinker. As the calcination temperature increases, the amount of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ produced gradually increases. At temperatures above 1300 °C, the decomposition of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ leads to the formation of C₁₂A₇ and CA.

(3) The clinker mineral ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$ is in the form of round grains and hexagonal plates with a crystal size of 1 to 2 μm, which is relatively tiny and favors the hydration of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$. Calcination temperatures above 1300 °C can affect the particle morphology of ${\mathrm{C}}_4{\mathrm{A}}_3\overline{\mathrm{S}}$, changing it from a well-defined polygonal structure to a molten state.

Further studies on the doping behavior of PG in the preparation of P and F elements are needed. In addition, only the compressive strength of the prepared CAS was investigated in this paper; work on other working properties (solidification time, bulk stability, etc.) will follow.